Hydraulic cement has a wide variety of applications in which it provides positioning and structural integrity. The ability to detect the location and environmental condition of cement can be used to detect or prevent failure and improve the design and performance of structures made out of it.
Cementing is a crucial step in drilling safely for oil and natural gas. However, it can be prone to frequent failures which compromise well integrity, productivity, and safety. Also this important barrier provides isolation of production phases within the well and separation of the well and the aquifer. Cement sheath integrity is a key factor controlling the life of the well. Risks to well integrity must be addressed and mitigated up-front.
Acoustic interrogation techniques that can be applied to cement and geophysical measurement and monitoring range in frequency from sub-sonic (or seismic) to ultra-sonic. They typically involve use of a sound source and a series of detectors that measure acoustic reflections. The sound wavelengths largely determine the range and resolution of measurements. Advanced signal processing can be applied to the response to allow for improved acoustic mapping. Fiber optic cable can be used for distributed acoustic sensing, providing additional opportunities for acoustic analysis. In the case of cemented oil and gas wells, aside from acoustic stimulation from within the wellbore fluid, the casing itself can be stimulated by a mechanical or electromagnetic-acoustic means as a sound source for acoustic interrogation. These methods can also be used to create acoustic relays to convey information along the wellbore.
Conventional techniques to inspect the integrity of cementing behind multiple casing strings are inaccurate, insufficient, and not reliable. Traditional sonic cement bond logs provide some information regarding the bond between the cement and the casing, but provide little information about the cement itself and are often difficult to interpret. Existing evaluation techniques have been used for over thirty years and provide little accuracy. One major problem is that lightweight or contaminated cement has similar acoustic impedance as surrounding fluids, making it difficult to distinguish. Additional acoustic methods for well cement and formation evaluation include cross-well tomography, full waveform imaging, pulse-echo techniques, and flexural wave propagation measurements.
Sound sources for seismic measurements include sparkers, boomers, and chirp transducers and cover a range of lower frequencies. These sources provide good resolution to about 30 feet and lower resolutions up to 100's of feet. Traditional downhole sonic logging tools have a depth of penetration of about one source wavelength, typically 1 to 5 feet. Borehole acoustic reflection surveys use seismic processing methods with a downhole tool to map fractures up to 15 feet away from the wellbore. Cross-well seismic imaging provides similar ranges and resolutions as surface seismic, but measure across the formation instead of down into it. Improved materials for and methods of cement evaluation are needed to improve the range and resolution of such measurements.
Needs exist for improved cement evaluation technologies and approaches to improve the effectiveness of existing technologies to assess cement integrity.